Theoretical and developmental aspects on dental bond strength tests

Abstract

Modern dentistry relies on adhesion system, such that a good adhesion quality contributes to desirable treatment outcome. Thus, testing of the adhesion quality for dental structure is essential and important. Various tests, such as shear bond strength test (SBS), tensile bond strength test (TBS) and push out bond strength test (PO), were developed and widespread in use. However, bond strength tests are highly sensitive to the actual configurations (e.g. types of load and the geometries) of the specific application, such that various factors (e.g. sample preparation and artificial aging) can also affect the test results. Therefore, direct comparison of test results are often irrelevant.

In particular, SBS test, despite of its popularity, has been challenged and criticised recently since the test could create uneven stress at the bonding interface, i.e. “true” shear is not obtainable at the interface. Nevertheless, limited studies have revealed the correlations between the test configuration, stress distributions and the bond strength results. Thus, this study has used finite element (FE) method to theoretically compare and understand the stresses developed in the various test setup of bond strength tests. Then, FE was used to optimize different geometry parameters, e.g. size and loading

distance, for conventional SBS and new SBS tests, namely mold-enclosed shear bond strength test (ME-SBS) and lever-induced mold-enclosed shear bond strength test (LIME-SBS). Finally, our new protocol for laboratory test was investigated so as to overcome the said shortcomings.

The FE from this study revealed that, in general, the ME-SBS, LIME-SBS and TBS tests could provide a more even stress distribution than the conventional SBS test, whereas a closer load applied on the interface could reduce the bending moment of SBS test which would be useful in reducing uneven stress. The push out bond strength (PO) test also underwent uneven stress. The novel LIME-SBS was found to have minimal unnecessary tensile stress and maintain shear stress among all tests. For nearly all of the models, the periphery edge was under considerably higher stress than the inferior section of the bond area. In addition, every load and dimension gave very different stress results. Despite the laboratory bond strength results in this study were found to be ME-SBS > SBS > LIME-SBS, the theoretical tensile stress at the bonding area were found to be lowest in LIME-SBS.

Ultimately, due to the complexity of bond strength tests, it is suggested that every bond test setup should be accompanied with a FEA model to explain the stress distributions. Furthermore, the new LIME-SBS could be one regime to reduce the uneven stress under shear load.